Rep. Ron Paul is warning GOP voters that Rick Perry can’t be trusted after backing Al Gore for president, and the three-term Texas governor is pointing out that Mr. Paul thought President Reagan’s tenure was so bad that he ditched the party.

Former Utah Gov. Jon Huntsman Jr. says some of his rivals are too “extreme” to be elected, while Mr. Perry and former Massachusetts Gov. Mitt Romney trade barbs over their career choices and job-creation skills.

Mr. Reagan’s famous “11th Commandment” — Republicans shalt not speak ill of fellow Republicans — is being sorely tested in the heat of the 2012 presidential sweepstakes. Looking to distinguish themselves from rivals in the crowded, fluid race, GOP contenders are taking off the gloves and trading stiff punches on the campaign trail, questioning one another’s loyalty to the party and blasting one another’s records.

As the candidates gather Wednesday night for another debate, this time at the Ronald W. Reagan Presidential Library, Mr. Huntsman, for instance, is casting Mr. Perry and Rep. Michele Bachmann of Minnesota as too “extreme” to win a general election.

Mr. Romney is taking swipes at Mr. Perry’s decades-long tenure in elected office by blaming “career politicians” for the nation’s financial woes.

Mr. Perry, meanwhile, is hammering Mr. Romney’s record on jobs, saying that as governor of Massachusetts, he failed to create a job-producing environment.

The friendly fire coincides with the growing sense that President Obama is vulnerable in the 2012 election. But one Reagan scholar says the former president would not have minded the intraparty tussles that have erupted this year.

Craig Shirley, author of “Reagan’s Revolution: The Untold Story of the Campaign That Started It All,” said the “theology” behind the “11th Commandment” has been misunderstood. Since Mr. Reagan first embraced the philosophy during his 1966 gubernatorial bid in California, Mr. Shirley said, the media and politicians alike have twisted its meaning.

“People use it sometimes as a weapon to say, ‘You can’t talk about this Republican’s political views,’ but Reagan never meant it that way or interpreted it that way,” he said. “Reagan believed it was all fair game to talk about voting records, but getting into gratuitous personal attacks wasn’t.”

But Steven F. Hayward, author of “The Age of Reagan: The Conservative Counter-Revolution, 1980-1989,” said the trouble is that candidates often get caught in a “gray area” between personal and policy attacks.

“At what point does a personal attack and policy attack overlap?” he said, adding that it’s hard to expect a group of ambitious candidates to adhere to a literal reading of the philosophy. “That’s just the nature of politics,” he said. “The 11th Commandment runs smack into one of the iron laws of politics — that negative campaigning works.”

Both men said that Reagan’s 1976 presidential bid started to gain steam only after he attacked President Ford’s record, including his willingness to cede control of the Panama Canal. Although the notion was popularized by Mr. Reagan, it originated with California Republican Party Chairman Gaylord Parkinson, who hoped to avoid a repeat of the ugly Republican nomination race that contributed to Barry Goldwater’s landslide loss in the 1964 presidential election to Democrat Lyndon B. Johnson.

Although it is unclear whether history will repeat itself in 2012, it is clear is that the sniping between the GOP White House hopefuls is intensifying as they plunge into the post-Labor Day stretch of the primary season and gear up for a series of debates this month. Mr. Perry eventually will stand with the candidates on the same stage for the first time since entering the race nearly a month ago.

Since then, Mr. Perry has raced to the front in national polls, putting him squarely in the political cross hairs of his Republican rivals.

On Tuesday, Mr. Paul released a television spot that called into question Mr. Perry’s loyalty to the Republican Party. In it, the Paul camp contrasts Mr. Paul’s endorsement of Reagan at the 1976 Republican National Convention with Mr. Perry’s endorsement of Al Gore in the 1988 Democratic primaries.

“Al Gore found a cheerleader in Texas named Rick Perry,” the narrator says in the 60-second spot. “Rick Perry helped lead Al Gore’s campaign to undo the Reagan revolution, fighting to elect Al Gore president of the United States. Now America must decide who to trust. Al Gore’s Texas cheerleader or the one who stood with Reagan.”

In response, the Perry camp highlighted the resignation letter Mr. Paul wrote in 1987, in which he explained his reasons for cutting ties with the Republican Party, with much of the criticism aimed directly at the 40th president.

“There is no credibility left for the Republican Party as a force to reduce the size of government. That is the message of the Reagan years,” Mr. Paul wrote, blasting the deficits and debt run-up under the Reagan White House. (Mr. Paul ran for president in 1988 as the nominee of the Libertarian Party, but subsequently rejoined the GOP.)

Mark Miner, Perry campaign spokesman, called the Paul letter “a broadside attack on every element of President Reagan’s record and philosophy.”

“Paul thought President Reagan was so bad, he left the GOP,” Mr. Miner said. “It will be interesting to hear Rep. Paul explain why Reagan drove him from the party at tomorrow’s debate on the grounds of the Reagan Library.”